Small antenna
FIELD OF THE INVENTION 
     This invention relates to a small antenna for use with a portable small radio apparatus or the like. 
     BACKGROUND OF THE INVENTION 
     In the past it has been well known to provide a loop antenna consisting of a wire rod or loop antenna consisting of a metal strip inside a casing of a radio apparatus or the like. 
     These small antennas, however, have low radiation resistance, and so have extremely low efficiency, and the directivity of these loop antennas depends upon the shape of the loop antenna. Still more, the gain of the loop antenna depends upon the area of the loop, which is under the limitation of the shape or size of the casing, so that it is difficult to make a radio apparatus more portable and smaller without reducing the gain of the antenna. 
     BRIEF DESCRIPTION OF THE INVENTION 
     This invention has been made with a view to overcoming the problems involved with loop antennas used for the portable radio apparatus presently available. 
     This invention, therefore, has as its principal object the provision of a novel loop antenna consisting of two rectangular parallel boards with side frames which is small, thin and light in weight. 
     It is another object of this invention to provide a small antenna whose efficiency is better than heretofore available. 
     It is a further object of this invention to provide a small antenna which can provide a selected directivity characteristic. 
     According to the invention, the small antenna is comprised of two planar rectangular conductive boards in parallel, dielectric frame provided between two feedIng points on the conduction boards along all the four edges at arbitrary positions along one edge of the two conductive boards and at least one conductor which short-circuits the two conductive boards. 
     The conductive boards and the dielectric frames comprise the casing of a radio apparatus itself as well as the small loop antenna, consequently this small antenna has improved efficiency and better directivity characteristics than a conventional loop antenna provided inside the casing of the radio apparatus. Further, by changing the position of the conductor which short-circuits the two conductive boards, it is possible to change the directivity characteristic. Still the gain of the loop antenna of the invention is higher than that of a conventional antenna. 
     Other objects advantages and novel features of the invention will become more apparent from the following detailed description of the invention in conjunction with the accompanying drawings in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a preferred embodiment of the invention. 
     FIGS. 2a, 2b, and 2c are graphs showing the direct characteristic of an antenna constructed according to the invention. 
     FIG. 3 is a chart illustrating spherical coordinates to explain the polarization directivity characteristic. 
     FIG. 4 is a graph showing the frequency dependence of radiation efficiency. 
     FIG. 5a and 5b illustrate the automatic switching of short circuit positions. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Preferred embodiments of this invention will now be described with reference to the accompanying drawings. 
     A first form of a small antenna according to this invention is illustrated in FIG. 1. 
     The small antenna in this embodiment has two planar rectangular conductive boards I and 2 disposed in parallel at an interval h which is selected to be small when compared with the wavelength of the working radio wave. Dielectric frame 3 is disposed between and around all opposite edges of conductive boards 1, 2 and the entire conductive boards 1, 2 and 4 dielectric frame pieces, 3 comprise a casing for a radio apparatus itself. 
     The casing of this embodiment is a rectangular parallelipiped having a length 1=80 mm, a width w=50 mm, and a heigth (the interval between the two boards) h=3.6 mm. Inside this casing are functional circuits for a radio apparatus. 
     At an arbitrary position in one of the edges of rectangular conductive boards i and 2 there is a feeding point, which in this embodiment is a corner D-D 1 . 
     At least one conductor 4 positioned between arbitrary positions along the edges of the two conductive boards I and 2 short circuit the two conductive boards I and 2. In this embodiment, conductors 4 may be provided at one or more selected corners A, A 1  ; B, B 1  ; and C, C 1 . 
     Conductive board I and 2 disposed in parallel act as a loop antenna by the existence of conductors 4 which shortcircuit the two conductive boards. 
     The dimensions 1, w and h and the position of the feeding point (for example D and D 1  in the first embodiment) and positions of the short-circuit points are determined by the desired characteristics of the antenna and by data experience, with the final decision being made by experimentation. Interval h between conductive boards i and 2 have an influence upon the gain of the antenna, and so long as the value h is sufficiently small when compared with the wavelength of a working radio wave, the antenna gain can be made higher with the value h. Accordingly the dimensions 1, w and h, the position of the feeding point and the positions for short circuit are decided taking into account the above mentioned matter. 
     FIG. 2(a) shows the polarized directivity characteristics of the electrical field along the z axis when one corner angle D-D 1  is a feeding point and a conductor 4 short-circuits corner angles A-A 1 . FIG. 2(b) shows the characteristics when D-D 1  is a feeding point and the conductor 4 provides a short-circuit between corner angles B-B 1 . And FIG. 2(c) shows the characteristics when D-D 1  is a feeding point and a conductor 4 is provided to short-circuit still another corner angle C-C 1 . The symbol φ denotes the angle from the X axis (see FIG. 8). It is apparent from FIG. 2 that if the short-circuit position of conductive boards i and 2 disposed in parallel is changed at will, it is possible to change the polarization directivity characteristic of the electric field and, if the short-circuit position is changed automatically, it is possible to keep the directivity of the antenna at the most desirable condition responsive to the direction of a radio wave. 
     FIG. 4(a)-(c) shows the frequency dependence of radiation efficiency, when two points A, A 1  ; B, B 1  : or C, C 1 , respectfully are short-circuited. A resonant frequency is not changed when A, A 1 , B, B 1 , C, C 1 , or any other points are short-circuited, and radiation efficiencY is independent of the short-circuit position. 
     FIGS. 5a and 5b illustrate short circuit positions changed automatically by switching. In FIGS. 5 a and 5b elements 6, 7, such as pin diodes or capacitance variable diodes for short circuiting at high frequencies, short circuit corner angles A-A 1 , C-C 1  respectively. Radio receiver 8 provides a field strength detecting circuit (Receiving Signal Strength Indicator) which receives a signal from feeding points D-D 1  and generates an output signal at 81. Control circuit 5 successively short circuits short circuit elements 6, 7 and compares output signal 81 and selects the particular position short circuit elements 6, 7 that provide the maximum signal value at 81. 
     Actual measurements have confirmed that the present embodiment of the invention illustrated in FIG. 1 provides high efficiency as good as or better than the radio apparatus antennas of prior art. 
     Having described a specific embodiment having short-circuit points at A, A 1  ; B, B 1 , or C, C 1  to make it easy to understand it has been confirmed that the same effect is, as above achieved, when any other points are the short-circuit points and even when several points are simultaneous short-circuit points, or conductor 4 is a metal strip having a certain width. 
     This invention is not to be limited by the embodiment shown in the drawings and described in the description which is given by way of example and not of limitation, but only in accordance with the scope of the appended claims.